1. Field of the Invention
The present invention relates to a retardation compensation system that is effectively applied to a liquid crystal projector having three liquid crystal devices for full color display, and a liquid crystal projector having the retardation compensation system.
2. Description Related to the Prior Art
A three-panel type liquid crystal projector has three liquid crystal devices for red, green and blue images. The liquid crystal devices modulate red, green and blue light in accordance with three color images to form red, green and blue image lights. The red, green and blue image lights are combined in a combination prism, and focused on the screen through a projection lens system, so that many people can view a full color image at the same time. The above operation of the liquid crystal projector is the same, although the optical system is different in accordance with whether the liquid crystal projector is a front projection type or a rear projection type, and with whether the liquid crystal device is a transmissive type or a reflective type. The front projection type projector projects the image from the front side (viewer side) of the screen, while the rear projection type projector projects the image from the rear side of the screen.
Although various types of liquid crystal devices are available to the liquid crystal projector, TN (Twisted Nematic) type liquid crystal device is mainly used. The TN type liquid crystal device has a liquid crystal layer between a pair of substrates. The orientation of major axes of liquid crystal molecules in the liquid crystal layer is kept parallel to the substrates, and inclined gradually in the thickness direction so that the major axes of the liquid crystal molecules twist smoothly by 90 degrees along a path from one substrate and the other substrate. The liquid crystal molecule layer is sandwiched by a pair of polarizing plates (polarizer and analyzer). The polarization axes of the polarizing plates for a normally white liquid crystal device are perpendicular to each other (cross nicol configuration). The polarization axes of the polarizing plates for a normally black liquid crystal device are parallel to each other (parallel nicol configuration).
The liquid crystal device can display an image by use of its optical rotatory effect. In the normally white type liquid crystal device, incident light is linearly polarized by the first polarizing plate. When no voltage is applied to a non-selected pixel in the liquid crystal device, the liquid crystal molecules in the liquid crystal layer are twisted so as to rotate the polarization direction of linearly polarized light by 90 degrees. Linearly polarized light through the liquid crystal layer can pass the second polarizing plate, so that the non-selected pixel appears as a white state. When a certain level of voltage is applied to a selected pixel, twisted alignment of the liquid crystal molecules does not appear. In that case, the polarization direction of linearly polarized light is not rotated in the liquid crystal layer, so linearly polarized light is blocked by the second polarizing plate. Thus, the selected pixel appears as a black state.
The liquid crystal device has the disadvantage of narrow viewing angle because of its birefringence. Birefringence becomes dominant as the applied voltage to the liquid crystal layer increases. Although incident light perpendicular to the liquid crystal device is completely blocked in the black state, the liquid crystal layer exhibits birefringence to oblique incident light to change linearly polarized light into elliptical polarized light. Since elliptical polarized light can pass the second polarizing plate, leakage of incident light causes the decrease in the black density of the selected pixel.
Such birefringence of the liquid crystal molecules is appeared at a state between the white and black states, so oblique incident light partially leaks. Thus, the contrast ratio of the image on the liquid crystal device decreases if viewed obliquely. Any type of the liquid crystal device has, more or less, such birefringence.
A direct view type liquid crystal display to observe the image directly has a retardation compensator for the purpose of decreasing birefringent effect. As the retardation compensator, “Fuji WV Film Wide View A” (trade name, hereinafter referred to as “WV Film”), manufactured by Fuji Photo Film Co., Ltd., has been in the market. A form birefringence layer with stacked thin films is used as the retardation compensator to prevent the decrease of the contrast ratio of the obliquely viewed image, as described in the publication, Eblen J. P., “Birefringent Compensators for Normally White TN-LCDs”, SID Symposium Digest, Society for Information Display, 1994, pp. 245–248. In addition, U.S. Pat. No. 5,638,197 describes a retardation compensator in which thin film is obliquely deposited on a substrate.
The retardation compensators described above are utilized to the direct view type liquid crystal display in which an observer right in front of the display panel observes the image at a distance more than the distance of distinct vision. In the direct view type liquid crystal display, the observer can adjust the contrast ratio of the image in the edge area by moving the eye positions slightly. If the image is observed by plural observers at the same moment, low contrast ratio area unlikely occurs because the distance between the displayed image and the observers is large enough to decrease the viewing angle.
In the liquid crystal projector, incident light through the liquid crystal layer is projected to the screen through a projection lens system. The observer can view the projected image on the screen. The contrast ratio of the displayed image decreases because of oblique incident light to the liquid crystal layer. Then, it is impossible to increase the contrast ratio of the projected image even if the observer tries to change the viewing angle. The projection lens system with large back focus can increase the contrast ratio of the projected image because such lens system decreases the incident angle of incident light to the liquid crystal layer. Such projection lens system, however, is disadvantageous in terms of making the projector smaller.
Accordingly, the technique to increase the viewing angle of the liquid crystal display is effective in order to solve the contrast ratio problem of the liquid crystal projector. For instance, U.S. Patent Application Publication No. 2002/0018162 and Japanese Laid-Open Patent Publications (JP-A) No. 2002-031782 describe the technique to increase the contrast ratio of the projected image by applying the retardation compensator to the liquid crystal device for the liquid crystal projector. The liquid crystal projector in U.S. Patent Application Publication No. 2002/0018162 describes organic materials, such as the WV Film, as the retardation compensator for the TN type liquid crystal device. The retardation compensator in JP-A No. 2002-031782 discloses a uniaxial birefringent crystal, such as single crystal sapphire and crystal. In addition, JP-A No. 2002-131750 describes a Discotic type liquid crystal as the retardation compensator.
The retardation compensators described above work as the form birefringence body to exhibit optical anisotropy effect depending upon the incident angle of oblique incident light. Such anisotropy effect of the retardation compensator can prevent the decrease in the contrast ratio of the projected image which is caused by oblique emanation light from the liquid crystal device with large emanation angle.
The organic retardation compensator tends to be colored by long exposure to light containing ultraviolet component. Intensity of the light source in the liquid crystal projector has to be higher than that of the direct view type liquid crystal display. Higher intensity of the light source causes excessive heat to the retardation compensator. The retardation compensator tends to be colored brown in 2000 to 3000 hours. Because of such low durability, it is difficult to utilize the organic retardation compensator to the home use liquid crystal projection TV.
The retardation compensator made of sapphire or crystal has great durability for long-term use, but the sapphire and crystal are expensive. Moreover, the cut surface and the thickness of the sapphire or crystal must be controlled precisely to exhibit desired optical characteristics. Furthermore, the orientation of the retardation compensator of sapphire or crystal must be aligned precisely in the assembly of the projection optical system. Accordingly, sapphire or crystal retardation compensator is not appropriate for household type liquid crystal projector in terms of manufacture cost, regardless of great durability.
Although the liquid crystal projector has three liquid crystal devices for the purpose of displaying a full color image, the structure of the retardation compensator is the same regardless of the color. When the liquid crystal projector adopts the retardation compensator for green light, phase retardation of red and blue light may not be compensated appropriately. In that case, the intensity of the black state pixel increases so that the contrast ratio of the pixel image decreases. Moreover, the black pixel may be colored because of the change in the color balance.